[1] Univ Wisconsin, Dept Mat Sci & Engn, Madison, WI 53706 USA
[2] Univ Calif Berkeley, Dept Phys, Berkeley, CA 94720 USA
[3] Univ Calif Berkeley, Dept Mat Sci & Engn, Berkeley, CA 94720 USA
[4] Penn State Univ, Dept Mat Sci & Engn, University Pk, PA 16802 USA